Scroll type compressor having gradually thinned wall thickness

ABSTRACT

A scroll type compressor has a stationary scroll (1) and a movable scroll (2) rotating around the former in an orbital manner, while forming volume variable sealed spaces therebetween to compress a coolant gas. To reduce a wall thickness of the scroll, an outer wall curve (E 1   + ) of the scroll (1,2) is defined by the modification of a basic involute curve (D + ) by reducing a certain value Bθ n ) from a length (L 0 ) of the respective involute line of the basic involute curve, which value is increased as the involute angle (θ) is developed, and an inner wall curve (E 1   - ) is generated from the outer wall curve (E 1   + ) by first transferring the respective point (p 3 , 4) on the outer wall curve (E 1   + ) in the normal direction to the outer wall curve (E 1   + ) at the respective point (p 3 , 4) by a distance (r) equal to a radius of the orbital circle (R) to form an intermediate curve (E 1 ) and then symmetrically transferrring the respective point (p 5 ) on the intermediate curve (E 1 ) around the center (O 1 ) of the basic circle (C 1 ).

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a geometrical shape of a spiral bodybuilt-in to a scroll type compressor suitable for use in an automobileair-conditioner.

2. Description of the Related Arts

It is preferable to thin a wall thickness of a spiral body (hereinafterreferred to as "scroll") to reduce the weight of a scroll typecompressor, but the scroll is subjected to a severe counteraction of avarying compression of a medium gas. This is particularly true of thestart area of the scroll, as this area is exposed to a maximum pressure,and accordingly, at least this area of a scroll should have a wallthickness sufficient to withstand such a pressure and avoid damage dueto wear. In the conventional scroll type compressor, an involute curveis used as a profile of outer and inner walls of both the movable andstationary scrolls, and therefore, the wall thickness is uniform overall of the wall length. Accordingly, if the start area of the scroll hasa sufficient wall thickness, it continues to the end area thereof, andthus a thinning of the scroll wall becomes impossible.

A solution is proposed in Japanese Unexamined Patent Publication (Kokai)No. 60-98186, in which a wall thickness of a movable scroll is graduallyreduced toward an end area thereof, and a wall thickness of a stationaryscroll is increased correspondingly. Profiles of both the outer andinner walls are involute curves, and a basic circle of the outer wallcurve has a smaller diameter than that of a basic circle of the innerwall curve. The use of the basic circles, each having a differentdiameter, enables the wall thickness of the movable scroll to be madethinner toward the end area thereof. The reduction of the wall thicknessof the movable scroll is compensated by the increase of that of thestationary scroll, so that a smooth contact between both scrolls can beensured during the orbital motion of the movable scroll.

According to the above-mentioned proposal, nevertheless the weight ofthe movable scroll is reduced when enhancing the mechanical strength ofthe start area thereof, the weight of the stationary scroll isconversely increased, and therefore, the total weight of the compressorcannot be reduced. Further, as the profiles of the outer and inner wallare still involute curves, a reduction of a diameter of the scrollcannot be attained, which is essential to the compactness of this typeof compressor.

SUMMARY OF THE INVENTION

Thus, an object of the present invention is to provide a compressor withscrolls having an improved shape by which a total weight and the size ofthe compressor are reduced.

This object can be achieved by a scroll type compressor comprising astationary scroll and a movable scroll, outer and inner walls of themovable scroll confronting those of the stationary scroll and beingsupported to be subjected to an orbital motion along an orbital circlewhile prevented from spinning around its own axis, a sealed space beingformed between both the scrolls, which is reduced in volume when themovable scroll is subjected to the orbital motion, profiles of walls ofboth the scrolls being defined by a curve generated from themodification of an involute curve of a basic circle, characterized inthat a wall thickness of the stationary and movable scrolls is graduallythinned from the start area to the end area of the scrolls.

More specifically, a scroll type compressor according to the presentinvention is characterized in that the curve defining a profile of theouter wall (outer wall curve) is generated from a basic involute curveby lowering a certain value from a length of the respective involuteline of the basic involute curve, which value is increased as theinvolute angle is developed; the curve defining a profile of the innerwall (inner wall curve) is generated from the outer wall curve by firsttransferring the respective point on the outer wall curve substantiallyin the normal direction to the outer wall curve at the respective pointby a distance equal to a radius of the orbital circle to form anintermediate curve, and then symmetrically transferring the respectivepoint on the intermediate curve around the center of the basic circle;wherein the involute line is defined by a segment of tangent to thebasic circle at the respective involute angle, between the involutecurve and the basic circle.

Preferably, in the generation of the intermediate curve, the respectivepoint on the outer wall curve is transferred correctly in the normaldirection.

Alternatively, in the generation of the intermediate curve, therespective point on the outer wall curve is transferred in the directionof the involute line at the respective point.

BRIEF DESCRIPTION OF THE DRAWINGS

The other objects and advantages of the present invention will beapparent with reference to the preferred embodiments illustrated by thefollowing drawings:

FIGS. 1 through 4 are schematic views, respectively, illustrating asequential change of the contact between stationary and movable scrolls;

FIGS. 5 through 7 are schematic views, respectively, illustrating asequence of a procedure for the generation of curves defining profilesof outer and inner walls of the scroll according to the presentinvention; and

FIGS. 8 and 9 are schematic views, respectively, illustrating thecontact between outer and inner walls of the stationary and movablescrolls according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 1 through 4 represent, respectively, a sequential change of thecontact between a stationary scroll 1 and a movable scroll 2 when themovable scroll 2 moves at an angular pitch of 90° on its orbital circle.According to the orbital motion of the movable scroll 2, the volumes ofa plurality of sealed spaces S₁, S₂, S₃ and S₄ between both scrolls 1and 2 are gradually reduced so that a gas therein is compressed. In FIG.2, spaces S₁ and S₂ are communicated with a discharge port 3 and the gasis discharged therefrom as shown in FIGS. 3 and 4. Thereafter, the nextsealed spaces S₃ and S₄ are communicated with the discharge port 3 andthe same steps are repeated.

Curves E₁ ⁺ and E₁ ⁻ defining, respectively, profiles of outer and innerwalls of the stationary scroll 1 and curves E₂ ⁺ and E₂ ⁻ defining,respectively, profiles of outer and inner walls of the movable scroll 2are not the conventional involute curve but are modified so that a wallthickness of the respective scrolls 1, 2 is gradually thinned toward theend area thereof.

The curve depicted by a solid line in FIG. 5 is the abovesaid modifiedinvolute curve E₁ ⁺ of the outer wall of the stationary scroll 1, andcurve D⁺ depicted by a chain line is a pure involute curve generatedfrom a basic circle C₁ of radius A with a center positioned at an originO₁ of x--y coordinates. The starting point of this involute curve D⁺ isdefined at a point p₁ on x axis. R designates a circle having a radius requal to that of the orbital path of the movable scroll 2.

Curve D⁺ is represented by

    x.sup.2 +y.sup.2 =A.sup.2 +A.sup.2 θ.sup.2           (1)

wherein θ is an involute angle, a position corresponding thereto beingrepresented on the basic circle C₁ by a point p₂ in FIG. 5.

Aθ in equation (1) represents a length of an involute line correspondingto a segment between the point p₂ and a point p₃ which is anintersecting point of the involute curve D⁺ with a tangent 1₁ to thecircle C₁ at the point p₂. In general, the length of the involute lineL₀ is expressed as a function of θ by

    L.sub.0 (θ)=Aθ                                 (1')

The curve E₁ ⁺ defining the profile of the outer wall is represented by

    x.sup.2 +y.sup.2 =A.sup.2 +(Aθ-Bθ.sup.n).sup.2 (2)

wherein B is a positive constant and n is an exponent of more than two.

(Aθ-Bθ^(n)) in equation (2) represents a distance between the point p₂and a point p₄ which is an intersecting point of the tangent 1₁ with thecurve E₁ ⁺. In other words, Bθ^(n) represents a distance between thepoints p₃ and p₄, and the curve E₁ ⁺ is obtained by substrate Bθ^(n)from the length of involute line. Accordingly, the outer wall curve E₁ ⁺is gradually moved away inward from the involute curve D⁺ as theinvolute angle θ increases.

To simplify the drawing, a curve (D⁺, E₁ ⁺) in FIG. 6 commonlyrepresents the involute curve D⁺ or the outer wall curve E₁ ⁺ thusobtained. 1₂ is a tangent to the curve (D⁺, E₁ ⁺) at a point p₃,4 whichis an intersecting point of the tangent 1₁ at the involute angle θ withthe curve (D⁺, E₁ ⁺), and 1₃ is a normal to the curve (D⁺, E₁ ⁺) at thepoint p₃,4. While, a curve (D, E₁) is a concurrence of points p₅, eachdefined by transferring the point p₃,4 along the normal 1₃ by a distancecorresponding to a radius r of the orbital circle R. According to thistransfer, the starting point p₁ of the curve (D⁺, E₁ ⁺) is transferredto a point p₆. This curve (D, E₁) is referred to as an "intermediatecurve".

If x and y components of the distance r along the normal 1₃ are a_(x)and b_(y), respectively, r is defined by

    r.sup.2 =a.sub.x.sup.2 +b.sub.y.sup.2                      (3)

If the point p₅ has coordinates (X, Y), X, x and Y, y are related by

    X-x=a.sub.x

    Y-y=b.sub.y                                                (4)

The relationship between the points p₃,4 (x, y) and p₅ (X, Y) isexpressed by

    r.sup.2 =(X-x).sup.2 +(Y-y).sup.2                          (5)

From equations (4) and (5), the following is obtained:

    X.sup.2 +Y.sup.2 =x.sup.2 +y.sup.2 +r.sup.2 +2 (xa.sub.x +yb.sub.y)(6)

x and y are also expressed as a function of θ by

    x=A cos θ+Aθ sin θ

    y=-Aθcos θ+Asin θ                        (7)

As shown in FIG. 6, a_(x) and b_(y) are defined as a function of angle βformed between the normal 1₃ and a straight line 1_(y) passing the pointp₃,4 in parallel to y axis by

    a.sub.x =r cos (β-π/2)

    b.sub.y r sin (β-π/2)                              (8a)

when θ is in first and third quadrants, and

    a.sub.x=r cos (β+π/2)

    b.sub.y =r sin (β+π/2)                             (8b)

when θ is in second and fourth quadrants.

From equations (6), (7) and (8a), the following is obtained ##EQU1##

From equations (6), (7) and (8b), the following is obtained ##EQU2##

However, it is apparent that these two equations (9a), (9b) areidentical when the tangent l₁ and the normal l₃ are coincident with eachother with reference to the relationship of β=θ-π.

When the curve (D⁺, E₁ ⁺) is a pure involute curve D⁺, the normal 1₃ iscoincident with the tangent 1₁. This is proved as follows:

If coordinates of the point p₂ on the basic circle C₁ at an involuteangle θ is (x₀, y₀) a gradient dy₀ /dx₀ of the tangent 1₁ is defined by

    dy.sub.0 /dx.sub.0 =(y-y.sub.0)/(x-x.sub.0)                (10)

As x₀ =A cos θ and y₀ =A sin θ, the equation (10) is represented by

    dy.sub.0 /dx.sub.0 =-1/tanθ                          (11)

By differentiating the equation (1) for x, the following is obtained:

    x+y dy/dx=A.sup.2 θ dθ/dx                      (12)

By differentiating x in the equation (7) for θ, the following isobtained:

    dx/dθ=Aθ cos θ                           (13)

From the equations (12) and (13), the gradient dy/dx of the tangent of1₂ is represented by

    dy/dx=(A/cosθ-x)/y                                   (14)

By substituting x, y in the equation (14) by the equation (7), thefollowing is obtained:

    dy/dx=tanθ                                           (15)

The equation (15) shows that the tangents 1₁ and 1₂ intersect with eachother at a right angle. Thus, it is apparent from the equation (11)that, if the curve (D⁺, E₁ ⁺) is a pure involute curve D⁺, the gradientsof the normal 1₃ and the tangent 1₁ coincide with each other.

Accordingly, β is equal to (θ-π), and the equations (9a) or (9b) isconverted ##EQU3##

This equation (16) is simplified to

    X.sup.2 +Y.sup.2 =x.sup.2 +y.sup.2 +r.sup.2 +2rAθ    (17)

From the equations (1) and (17), the following is obtained:

    X.sup.2 +Y.sup.2 =A.sup.2 +A.sup.2 θ.sup.2 +r.sup.2 +2rAθ(18)

Substitution of r in the equation (18) by A α results in

    X.sup.2 +Y.sup.2 =A.sup.2 +A.sup.2 (θ+α).sup.2 (19)

This means that if the curve (D⁺, E₁ ⁺) is a pure involute curve D⁺, theintermediate curve (D, E₁) also becomes a pure involute curve D obtainedthrough the clockwise rotational transfer of the curve D⁺ around theorigin O₁ by an angle α. A profile of the conventional inner wall isdefined by an involute curve D³¹ in FIG. 7, obtained by the symmetricaltransfer, i.e., 180° rotational transfer of the intermediate curve Daround the center of the basic circle C₁. Accordingly, the curve D⁻ isalso obtained by the counterclockwise rotational transfer of theinvolute curve D around the origin O₁ by an angle (π+α).

As the normal 1₃ and the tangent 1₁ coincide with each other, thenormals 1₃ at the starting point p₁ (A, O) of the involute curve D isparallel to y axis, and the point p₁ is transferred in parallel to yaxis to the starting point p₆ (A, -r) of the curve D. The point p₆ isfurther transferred to a starting point p₇ (-A, r) of the curve D by thesymmetrical transfer around the origin.

An inner wall curve E₁ ⁻ corresponding to the outer wall curve E₁ ⁺defined by equation (2) is obtained in a similar manner as the case ofobtaining the involute curve D⁻ from the involute curve D⁺ describedabove. That is, first a curve E₁ is formed by transferring the outerwall curve E₁ ⁻ along the normal 1₃ at a distance corresponding toradius r of the orbital circle, and then the inner wall curve E₁ ⁻ isobtained by the symmetrical transfer of E₁ around the origin. A point p₈in FIG. 7 represents a position of a point p₅ (X, Y) on the curve E₁after the symmetrical transfer around the origin has been completed.

The curve E⁺ is represented by

    (X-a.sub.x).sup.2 +(Y-b.sub.y).sup.2 =A.sup.2 +(Aθ-Bθ.sup.n).sup.2                          (20)

The curve E₁ ⁻ is represented by

    (X+a.sub.x).sup.2 +(Y+b.sub.y).sup.2 =A.sup.2 +(Aθ-Bθ.sup.n).sup.2                          (21)

An outer wall curve E₂ ⁺ and an inner wall curve E₂ ³¹ of the movablescroll are identical to the outer and inner wall curves E₁ ⁺ and E₁ ⁻ ofthe stationary scroll, respectively. FIG. 8 illustrates the contactbetween the outer wall curve E₁ ⁺ of the stationary scroll 1 and theinner wall curve E₂ ⁻ of the movable scroll 2 and between the inner wallcurve E₁ ⁻ of the stationary scroll 1 and the outer wall curve E₂ ⁺ ofthe movable scroll 2. The inner and outer wall curves E₂ ⁻ and E₂ ⁺ ofthe movable scroll 2 are obtained by symmetrically transferring theinner and outer wall curves E₁ ⁻ and E₁ ⁺ of the stationary scroll 1around the origin, and further, transferring the resultant curves sothat the center of the basic circle C₁ is positioned on the orbitalcircle R. A circle C₂ in FIG. 8 is a basic circle of the outer wallcurve E₂ ⁺.

When a center O₂ of the basic circle C₂ coincides with a point p₉ (O, r)on the orbital circle R as shown in FIG. 8 by an imaginary line, astarting point p₁₀ of the outer wall curve E₂ ⁺ of the movable scroll 2coincides with the starting point p₇ (-A, r) of the inner wall curve E₁⁻ of the stationary scroll 1 and a starting point p₁₁ of the inner wallcurve E₂ ⁻ of the movable scroll 2 coincides with the starting point p₁(A, 0) of the outer wall curve E₁ ⁺ of the stationary scroll 1. Thebasic circle C₂ shown by an imaginary line having a center at the pointp₉ (0, r) is transferred to a position shown by a solid line so that thecenter thereof coincides with a point O₂ by the counterclockwiserotational transfer at an angle θ on the orbital circle R. Then straightlines p₂ -O₁ and O-O₂ intersect with each other at a right angle. If aposition at an involute angle θ on the basic circle C₂ shown by a solidline is a point p₁₂, straight lines O₂ -p₁₂ and O₁ -O₂ intersect witheach other at a right angle. Accordingly, a point p₁₃ on the outer wallcurve E₂ ⁺ of the movable scroll 2 corresponds to the point p₄ on theouter wall curve E₁ ⁺ of the stationary scroll 1 at an involute angle θ.The point p₁₃ does not coincide with the point p₈ on the inner wallcurve E₁ ⁺ of the stationary scroll 1 in FIGS. 7 and 8. This is becausethe gradient of the normal 1₃ at the point p₄ on the outer wall curve E₁⁺ is different from that of the tangent 1₁.

However, since the points p₈ and p₁₃ are distant from each other only inthe tangential direction on the curve E₁ ⁻ or E₂ ⁺ but the deviationtherebetween is almost zero in the normal direction, both the scrolls 1and 2 are considered to be in contact with each other in the closevicinity of the points p₄ and p₁₃. This can be explained as follows:

If x-component and y-component of Bθ^(n) are Δx and Δy, respectively,coordinates of the point p₁₃ (X, Y) are represented by

    X=x-Δx

    Y=y-Δy                                               (22)

As the gradient of the tangent 1₄ is -1/tanθ, Δx and Δy are expressed by

    Δx=Bθ.sup.n ·sinθ

    Δy=-Bθ.sup.n ·cosθ              (23)

By differentiating the equation (2) while substituting X, Y for x, y,respectively, the following is derived:

    X+Y dY/dX=(Aθ-Bθ.sup.n)(A-Bnθ.sup.n-1)dθ/dX(24)

By substituting (24) for (22), the following equation is derived:

    (x-Bθ.sup.n sinθ)+(y+Bθ.sup.n cosθ)dY/dX=(Aθ-Bθ.sup.n)(A-Bnθ.sup.n-1)dθ/dX(25)

From the equations (22) and (23), dX/dθ is obtained as follows: ##EQU4##

If n=2 and θ=π, for example, the following equation is obtained from(25) and (26):

    dY/dX=2B/(A-Bπ)                                         (27)

The equation (27) represents the gradient of tangent on the basic circleC₂ at an involute angle π. If A=0.5 cm and B=0.001, dY/dX is 0.004.While, according to the equation (15), dy/dx is 0. The differencetherebetween is substantially on the same order at other involuteangles. That is, an intersecting angle Δθ between the normals at pointsp₁₃, p₈ is nearly equal to 0.004 radian. This means that, when theorbital radius r is 1 cm, the distance between points p₁₃ and p₈ has atangential component of 0.004×1 cm=0.004 cm and a normal component of0.004 cm×0.004=0.000016 cm. The normal component of 0.000016 cm iswithin a manufacturing tolerance of the scroll wall. Accordingly, theinner and outer wall curves E₁ ⁻, E₁ ⁺ of the stationary scroll 1 andthe inner and outer wall curves E₂ ³¹ , E₂ ⁺ of the movable scroll 2 canbe substantially always in contact with each other when the movablescroll 2 is subjected to an orbital motion.

The outer wall curve E₁ ⁺ expressed by equation (2) is also representedby

    L.sub.1 (θ)=Aθ-Bθ.sup.n                  (28)

Similarly, the inner wall curve E₁ ⁻ defined by equation (21) is alsorepresented by

    L.sub.2 (θ)=A(θ-π)-B(θ-π).sup.n    (29)

As shown in FIG. 7, a wall thickness t of the stationary scroll 1 in thedirection of tangent 1₄ on the basic circle C₂ of the inner and outerwall curves E₁ ⁻ and E₁ ⁺ is represented as follows:

    t(θ)=L.sub.1 (θ)-L.sub.2 (θ-π)        (30)

If n=2, the equation (30) is converted to

    t(θ)=Aπ-2Bθπ+Bπ.sup.2                 (31)

That is, the wall thickness t is linearly reduced as an involute angleis increased. This is also true for the case in which n is more thanthree. Accordingly, the start area of the scroll wall subjected to asevere high pressure is strengthened by increasing the wall thicknessand the end area thereof not subjected to such a high pressure can bethinned, whereby the weight of a compressor can be reduced.

As illustrated in FIG. 1, the stationary scroll has maximum involuteangle θ of about 11π/2 in the embodiment described. A length L₁ ofinvolute line corresponding to the involute angle θ of 11π/2 is about8.337 cm which is shorter than L₀ of 8.635 cm in the case of the pureinvolute curve D⁺. Since a radius of the stationary scroll 1 correspondsto this length L₁, it is apparent that a size of the compressor also canbe reduced.

According to this embodiment, as shown in FIG. 9, a starting point p₁ ofthe outer wall curve E₁ ⁺ and a starting point p₇ of the inner wallcurve E₁ ⁻ are smoothly connected by a curve F not invading the orbitalcircle R.

The present invention is not limited to the above embodiment. When aninner wall curve is generated from an outer wall curve, points on theouter wall curve may not be shifted strictly in the normal direction butin the approximately normal direction. For example, they may be shiftedin the direction of the involute line provided a coefficient B isproperly modified. The resultant curves are smoothly in contact witheach other.

We claim:
 1. A scroll type compressor comprising a stationary scroll anda movable scroll, outer and inner walls of the movable scrollconfronting those of the stationary scroll and being supported to besubjected to an orbital motion along an orbital circle while preventedfrom spinning around its own axis, a sealed space being formed betweenboth the scrolls which is reduced in volume when the movable scroll issubjected to the orbital motion, profiles of walls of both scrolls beingdefined by a curve generated from a modification of an involute curve ofa basic circle, characterized in thatthe curve defining a profile of theouter wall (outer wall curve) is generated from a basic involute curveby reducing a certain value from a length of the respective involuteline of the basic involute curve, which value is increased as theinvolute angle is developed; and the curve defining a profile of theinner wall (inner wall curve) is generated from the outer wall curve byfirst transferring the respective point on the outer wall curvesubstantially in the normal direction to the outer wall curve at therespective point by a distance equal to a radius of the orbital circleto form an intermediate curve and then symmetrically transferring therespective point on the intermediate curve around the center of thebasic circle; wherein the involute line is defined by a segment oftangent to the basic circle at the respective involute angle between theinvolute curve and the basic circle.
 2. A scroll type compressor asdefined by claim 1, characterized in that, in the generation of theintermediate curve, the respective point on the outer wall curve istransferred correctly in the normal direction.
 3. A scroll typecompressor as defined by claim 1, characterized in that, in thegeneration of the intermediate curve, the respective point of the outerwall curve is transferred in the direction of the involute line at therespective point.
 4. A scroll type compressor as defined by claim 1,characterized in that the profile of the outer wall curve is defined onx-y co-ordinates by the following equation

    X.sup.2 +Y.sup.2 =A.sup.2 +(Aθ-Bθ.sup.n).sup.2

wherein A is a radius of the basic circle, B is a positive constant, nis an exponent of more than two, and θ is an involute angle.